Phenolphthalein, a constituent in many laxatives, showed clear evidence of carcinogenic activity in 2-year feeding studies in rodents done by the National Toxicology Program (NTP). Phenolphthalein-containing laxatives, which have been shown to be carcinogenic in laboratory animals at doses only 10-fold greater than those frequently consumed by humans, have recently been on the market and widely used. About 3% of the older U.S. population uses phenolphthalein-containing laxatives on a daily basis. We investigated the relation of phenolphthalein-containing laxative use with risk of ovarian cancer and adenomatous colorectal polyps in humans. Ovarian cancer was one of the specific forms of cancer observed in the NTP animal, but few epidemiologic studies have examined ovarian cancer risk in relation to laxative use. Therefore, we conducted two case-control studies to evaluate the relation of phenolphthalein-containing laxative use with risk of ovarian cancer. Neither study showed strong evidence of an association.